1. Field of the Invention
This invention relates to a document feeding device for automatically feeding and discharging documents to and from image processing devices of various sorts, and more particularly to an automatic document feeding device capable of transporting documents lying one upon another in order of page one by one at high speed from a document supply mechanism to an image scanning position of the image processing device and sending out the document from the image scanning position to a document discharge mechanism after completing image processing so as to resultantly put the discharged documents one on top of another in the same order of pages as the documents initially placed on the document supply mechanism.
2. Description of the Prior Art
There have been widely used systems having a document feeding device placed on the image scanning position of an image processing device such as a copying machine, facsimile and image scanner, which deals with documents or originals printed with characters and/or graphics, to automatically carrying out consecutive processes of feeding and discharging the documents one by one relative to the image processing device.
The documents placed on a document supply tray of the document feeding device are sent off one by one toward the image scanning position of the image processing device so as to be sent out in the sequence of the uppermost document to the lower documents or vise versa of the documents stacked on the tray. When feeding the document to the image scanning position, there is a case that the document must be turned over according to the state in which the image surface of the document on the document supply tray faces upward or downward.
An explanation of the sequence of transporting documents by the document feeding device will be given with reference to FIG. 1 shown by way of one example. The document feeding device ADF is openably mounted on an image processing device M so as to be overlaid on a platen made of transparent glass or the like which serves as the image scanning position defined on the upper surface of the image processing device. The document feeding device comprises a document supply mechanism 1 for sending off documents Dn (n.gtoreq.1) stacked on a document supply tray 1a, a document transport unit 2 for moving the document from the document supply mechanism 1 to an image scanning position R defined on the platen, and a document discharge mechanism 3 for sending out the document from the image scanning position R to a document discharge tray 3a.
The documents D1 to D3 placed on the document supply tray 1a are sent off one by one and introduced into a supply passage p1 by rotating a feed roller 1b in the document supply mechanism 1, so that one document is fed into the image scanning position R defined between the document transport unit 2 and the platen P. The document after being imaged at the image scanning position R is discharged to the document discharge tray 3a through a delivery passage p2.
The document supply tray 1a is disposed above the document discharge tray 3a for the purpose of increasing a space factor of the device. Therefore, when the documents Dn are stacked on the document supply tray 1a with their image faces f downward as illustrated, and at the outset, the lowermost first document D1 is first fed to the image scanning position over the platen P through the supply passage 1a of the document supply mechanism 1, the image face of the document arriving at the image scanning position faces upward as indicated by dx1 in the illustration. Thus, the document to be fed to the image scanning position should be turned upside down.
To turn over the document, the document is usually moved through a round route formed of the delivery passage p2 and a return passage p3 and sent back to the upper surface of the platen, as indicated by the arrow a in the illustration. Otherwise, the document once arriving at the image scanning position may be sent back to the document supply passage p1 of the document supply mechanism 1 to be turned over there.
The document dx2 sent back to the upper surface of the platen through the return passage p3 is placed there with the image face f downward while locating the leading end thereof at a scanning reference point PX. In this state, the desired image processing is carried out.
The document after being imaged at the image scanning position is discharged to the discharge tray 3a through the delivery passage p2. However, if the document is discharged as it is, the image face f of the document faces upward on the discharge tray 3a, and succeeding documents D2 and D3 are similarly sent out on the top of the document D1 on the discharge tray 3a. As a result, the order of page of the documents discharged to the discharge tray is reverse to that of the documents initially stacked on the document supply tray. To surmount such a disadvantage, the document dx2 after being imaged pursues the same course indicated by the arrow a to move through the delivery passage p2 and the return passage p3 to the upper surface of the platen, and then, be sent out through the delivery passage p2 and a discharge passage p4 to the discharge tray 3a. Consequently, the documents Dn can be discharged to the discharge tray 3a with their image faces downward in the same order of page as those initially stacked on the document supply tray.
Even though the documents are initially placed with their image faces upward and sent out in sequence from the uppermost document, the documents after being imaged are inevitably required to be turned over as shown by the arrow a in the illustration.
Since the document feeding device cannot however determine a lead-in distance by which the document must be returned onto the upper surface of the platen along the returning course indicated by the arrow a in the illustration, there will be commonly thought out an idea of returning the document delivered through the returning course until the leading end of the document arrives at the scanning reference point PX (position at which the document dx2 is placed to be imaged), and yet, the document of a smaller size than the maximum size which can be dealt with by the document feeding device must be excessively returned more than it needs, thereby decreasing the feeding efficiency and reducing the operation speed of the document feeding device, because the platen is generally designed according to the size of the maximum document. That is, in some general cases, the image processing device such as a copying machine can deal with documents of A3-size (29.7 cm.times.42 cm) at a maximum, but documents of A4-size or letter size smaller than the maximum size would be usually used with considerable frequency. Hence, it is irrational that the document of A4-size is transported from one end to the other end of the image scanning position having a longitudinal length equal to or larger than A3-size, as a result of which the operation speed of the document feeding device is disadvantageously reduced.